Method of drilling and jet drillilng system

ABSTRACT

A method of drilling into an object comprises providing a drill string in a borehole in the object, the borehole having at its bottom a borehole axis, the drill string comprising a borehole-centralized jet drill head at its lower end, the jet drill head having a drill axis, and comprising a jet nozzle; providing a stabilizer means for the drill string, the stabilizer means determining an inclination angle between the borehole axis and the drill axis; and generating a fluid jet at the jet nozzle, so as to deepen the borehole, wherein the drill string is rotated while deepening the borehole. Another system comprises a drill string with an jet drill head at its lower end, the drill head being provided with a jet nozzle, a stabilizer positioned above the jet drill head, the stabilizer contacting the borehole wall, and means for rotating the drill string.

The invention relates to a method of drilling into an object, in particular by jet drilling, and to a jet drilling system. The object can in particular be a subsurface earth formation.

In the course of making a borehole, it is often desirable to control the drilling direction so as to provide a borehole along a predetermined trajectory. In conventional mechanical drilling, a common technology is to use equipment like bent subs, mud motors and rotating seals, to set only the lower part of the drill string with the drill bit to drill in a particular direction. Conventional mechanical drilling uses drilling bits with mechanical cutters such as roller-cones or polycrystalline diamonds, that produce cuttings by scraping at the borehole bottom and at the sides. More recently, rotary steerable systems (RSS) have been developed, which can operate with the entire drill string rotating. Known RSS methods point the mechanical drill bit into a desired direction using a complex bending mechanism, or push the drill bit to a particular side using expandable thrust pads. The side-cutting ability of the mechanical drill bits used for directional drilling then allows to deviate the borehole in the desired direction. For example, PDC bits have cutters not only on the front end but also at the sides. Known mechanical directional drilling systems are for example described in U.S. Pat. No. 5,168,941, U.S. Pat. No. 5,520,255, U.S. Pat. No. 5,857,531, U.S. Pat. No. 5,875,859, U.S. Pat. No. 6,092,610 and US2007/0163810.

Another type of drilling methods uses fluid jetting, instead of mechanical interaction between the drill bit or drill head and the object. However, directional drilling methods known from mechanical drilling cannot generally be used with jet drill systems. For example, equipment like bent subs, mod motors and rotary seals cannot be used at the high fluid pressures applied in jet drilling, which can be 100 bar or more, in particular 250 bar or more, more in particular 350 bar or more. Moreover known jet drilling heads bit do not have the side-cutting action that is needed for known mechanical directional drilling to work. Furthermore, abrasive jet drilling requires only minimal or even no weight on bit, which makes such drilling system react totally differently than conventional mechanical directional drilling methods and devices.

A jet drill system and method of making a hole in an object is disclosed in WO-A-2005/005767. The known system comprises an excavating tool, herein also referred to as abrasive jet drill head, mounted on a lower end of a drill string that is inserted from the surface into a hole in a subterranean earth formation. The drill string is provided with a longitudinal passage for transporting a drilling fluid mixture comprising abrasive particles to the drill head. The drill head comprises jet means arranged to generate an abrasive jet in a jetting direction into impingement with the earth formation in an impingement area. The abrasive jet contains magnetic abrasive particles (steel shot). A recirculation system is provided, which captures abrasive particles from the return stream to surface, after erosive impingement, by means of a magnet, and re-mixes the abrasive particles at a mixing location with the mixture received via the drill string. The magnet is arranged as a rotatable conveyor, attracting particles to be recycled and conveying them towards a mixing location with fresh fluid from surface. In the known system directional drilling is achieved by a modulation means in form of a controllable drive means for the conveyor, which is arranged so as to modulate the recirculation rate, and in this way the quantity of particles in the abrasive jet at the jet means is modulated. When the abrasive jet is moved along a trajectory in the hole, in particular in a rotating motion, the amount of erosion in each impingement area along the trajectory can be selectively varied, and directional control is achieved. Reference is also made in this regard to WO 2005/05766.

The control of directional effect via the recirculation system is relatively complex, and in fact requires the presence of abrasive particles in the fluid jet as well as a recirculation system.

There is a need for a new method of directional drilling for fluid jet drilling, which can be used independent of the presence and/or operation of a recirculation system for abrasive particles in the fluid jet.

In accordance with the present invention there is provided a method of drilling into an object, the method comprising

-   -   providing a drill string in a borehole in the object, the         borehole having at its bottom a borehole axis, the drill string         comprising a borehole-centralized jet drill head at its lower         end, the jet drill head having a drill axis, and comprising a         jet nozzle;     -   providing a stabilizer means for the drill string, the         stabilizer means determining an inclination angle between the         borehole axis and the drill axis; and     -   generating a fluid jet at the jet nozzle, so as to blast with an         erosive power on an impingement area of the borehole, thereby         deepening the borehole, wherein the drill string is rotated         while deepening the borehole.

The invention is based on the insight that directional tendency in a low weight-on-bit (WOB) jet drill system without side-cutting action is mainly determined by the angle between the drill axis and the borehole axis. Setting this angle to a non-zero value by a suitable stabilizer means is relatively easy in view of the low or virtually zero weight in bit. A stabilizer arranged on the drill string above the jet drill head can deviate the drill string slightly and sufficiently to form an inclination angle with the borehole, so that the fluid jetting builds a curved borehole. The rate at which the inclination angle of the drill bit increases depends on said angle. The better the drill string is centralized, the smaller this tendency is. The expression ‘stabilizer means’ is used to refer to a structure or device that protrudes radially with respect to adjoining drill string parts so as to provide at least one point of contact with the borehole wall during drilling. The point of contact together with the borehole-centralizing property of the jet drill bit mainly define a direction of progression of drilling. The object into which is being drilled can in particular be a subsurface earth formation containing a rock layer, such as a layer of sandstone, limestone, granit, basalt, or combinations of such layers. In such a formation, drilling system as for example known from BE 1001450A3 is not usable, as it is designed for providing a tunnel in the soil, wherein soil is compacted during progression. This is not possible in a rock formation. The known drilling system is moreover inadequate in that is does not rotate, and in that there is no centralizing means for a jet drill head.

In a preferred embodiment the fluid jet is an abrasive fluid jet and the jet drill head is an abrasive jet drill head. An abrasive fluid jet is a jet of a fluid mixture comprising a concentration of abrasive particles, e.g. steel shot in an aqueous liquid such as water.

The borehole centralized jet drill head can have a centralization means such as an annular centralizer, suitably at or around the drill head, and in any event downhole from the stabilizer means. In one preferred embodiment, the jet drill head is a self-centralizing jet drill head, which by its design remains centralized in the borehole during operation and does not require additional centralizing means such as an annular centralizer. The jet drill head can be self-centralizing as well as suitable for jetting abrasive particles.

In a preferred embodiment the stabilizer means is fixed with respect to the drill string while deepening the borehole, i.e. when the drill string is rotated during operation for deepening the borehole, the stabilizer means rotates together with the drill string and does not need to be fixed against the borehole wall, and released as the drill string progresses.

In a particular embodiment the stabilizer means is an eccentric stabilizer means, preferably the eccentric stabilizer means is rotated along with the drill string while deepening the borehole, more preferably the outer dimensions of the eccentric stabilizer means substantially conform to the borehole.

An eccentric stabilizer means has a non-uniform radial extension around the drill string. When the drill string rotates with the eccentric stabilizer means, and the eccentricity is fixed, the inclination angle between drill axis and borehole axis changes periodically. When the eccentric stabilizer conforms to the borehole wall, the inclination angle circles about the borehole axis. Depending on the relative direction of the jetting nozzle and the eccentricity, this way a straight borehole of increased (“overgauged borehole”) or in fact smaller diameter can be drilled when compared with a centralized drill string.

In certain embodiments the stabilizer means has a diameter smaller than the borehole diameter, preferably the largest diameter of the stabilizer means is smaller than the borehole diameter. When a borehole is somewhat deviated and no stabilizer is provided, an abrasive jet drill string has a natural tendency to build inclination angle, because the drill string due to the low weight on bit will be pulled to the lower side of the borehole under the influence of gravity. A stabilizer of smaller size than the borehole makes use of this effect, but defines the point of contact and therefore the inclination angle better.

In certain embodiments at least one radial dimension of the stabilizer means is adjusted in the course of a drilling operation, so as to deepen the borehole along a predetermined trajectory. A radial dimension is a dimension or direction perpendicular to the drill axis of the drill string. By changing a radial dimension the inclination angle can be influenced. When the stabilizer means is for example concentric with the drill string and smaller than the borehole, the inclination angle is set by setting the diameter of the stabilizer means arranged at a certain distance above the centralized jet drill head. When the stabilizer means is an eccentric stabilizer, the eccentricity can be changed, for example changing the borehole diameter that is being drilled.

In certain embodiments the stabilizer means has a cross-section that is adjusted while rotating the drill string, preferably wherein the stabilizer cross section is adjusted to maintain the drill axis geostationary during at least one rotation, more in particular wherein the geostationary drill string axis has a smaller angle with the vertical than the borehole axis. Such a stabilizer means can for example be an actively controlled means that maintains an eccentricity that is geostationary during rotation. The inclination angle can then be fixed, and in particular a negative inclination, i.e. an arrangement wherein the geostationary drill string axis has a smaller angle with the vertical than the borehole axis can be provided.

The invention moreover provides a jet drilling system for drilling a borehole into an object, comprising a drill string with an jet drill head at its lower end, the drill head being provided with a jet nozzle, a stabilizer means for the drill string at a position above the jet drill head, the stabilizer means protruding radially with respect to adjoining drill string parts so as to provide a point of contact with the borehole wall, and a rotation means for rotating the drill string.

The jet drilling system can further comprise centralizing means for the jet drill head downhole of the stabilizer means.

The jet drill had can have a front end for engaging the borehole bottom and an axis, and wherein the front end has a recess at least in an axial region. During operation of such a jet drill head, the undrilled material is present in the recess in the axial region, e.g. substantially in the form of a cone, and centralizes the jet drill head during rotation. Such a jet drill head is therefore self-centralizing. The front end engaging the borehole bottom can be a substantially circumferential front end. In the recess the jet nozzle can be arranged, preferably directed obliquely with respect to the drill axis, such as to form a cone in the recess during rotation.

The stabilizer means can be adjustable in at least one radial dimension, and to this end preferably further comprises an actuator for manipulating the at least one radial dimension. Such stabilizer means can comprise a plurality of pads the radial extension of which can be set by the actuator, e.g. hydraulically or with an electrical motor such as a stepper motor. An adjustable stabilizer is for example known from U.S. Pat. No. 4,572,305.

An adjustable jet drilling system preferably further comprises a control means for controlling the at least one radial stabilizer dimension, in particular downhole control means. Preferably the control means comprise a control signal receiver, such as a weight-on-bit sensor, a fluid pressure sensor, a rotational speed sensor.

In one preferred embodiment the jet drill head is an abrasive jet drill head.

In the following the invention will be described in more detail and by way of example also with reference to the accompanying drawings, wherein

FIG. 1 shows schematically a first embodiment of the invention; and

FIGS. 2-5 show schematically several further embodiments of the invention.

In the Figures, like reference numerals are used to refer to the same or similar objects.

In one aspect the invention is related to a jet drill method and system, in particular an abrasive jet drill system, comprising a rotatable drill string, a rotatable abrasive jet drill head connected to the lower end of the drillstring, the drill head being provided with at least one jet nozzle for discharging a mixture of drilling fluid and abrasive particles, drive means for rotating the drill bit and drill string and pump means for generating a flow of said mixture comprising drilling fluid and abrasive particles. Abrasive jet drill systems are known for example from WO 00/66872, WO 2002/034653, WO 2005/005766, W02008/119821, WO 2008/113843, WO 2008/113844.

By the combined action of the high pressure mixture jet which contains hard particles, and the rotational movement of the drill string and the drill bit, a borehole is formed in a rock formation as a result of the gradual erosion of the rock material. This erosion is solely obtained by the abrasive jet, without any mechanical cutting action of a cutter or drill bit, due to the high pressure drop over the jet nozzle of e.g. 250 bar or more, preferably 350 bar or more of. The abrasive particle concentration in the drilling fluid may be in the range of 2-10 vol %, such as 3 vol % or more. However, in case use is made of a downhole abrasive particle recycling device, the abrasive particle concentration passed from surface through the drill string may be as lower, such as 1.5 vol % or less while still giving a good penetration rate.

An aim of the present invention is to provide an abrasive jet drilling system wherein use is made of more simple devices and method steps for obtaining directional control. This aim can be achieved by means of at least one stabilizer at a position above the drill head, said stabilizer protruding radially with respect to the adjoining drill string parts. The influence of such stabilizer on the direction of the drilling process relies on the angle between the drill bit axis and the borehole axis. The stabilizer may rotate together with the drill string, in which case the stabilizer may extend around the drill string. For a fixed directional tendency, a stabilizer with a specific fixed radial dimension may be used. When a stronger tendency is required, a stabilizer with a reduced radial dimension or diameter should be used. As an example, a fixed spiral melon shaped drill collar stabilizer can be used. Such stabilizer can be positioned directly behind the abrasive jet drill head, possibly with a downhole abrasive particle recirculating device in between or integrated in the drill head.

Alternatively, the stabilizer may have an adjustable radial dimension. In that case, the directional tendency of the drilling system may be influenced by changing the radial dimension of the stabilizer. Such adjustable stabilizers can be embodied in several ways. For instance, in case the stabilizer comprises segments which are arranged next to each other in circumferential direction, these segments can be adjustable in radial direction. Preferably, said segments are adjustable independently from each other, e.g. by steering means such as stepper motors or hydraulic piston/cylinder devices.

In the latter embodiment, in case one of the segments is adjusted to a relatively large radial dimension and the opposite segment to a relatively small radial dimension, the axis of the drill head will show a non-zero angle with respect to the borehole axis. This adjustment may for instance be carried out at the earth surface and be kept during the ensuing drilling cycle. Alternatively however, downhole control means may be provided for controlling the stabilizer dimension. In the latter case, the direction of the borehole may be changed during the course of the drilling process, whereby a very flexible control of the drilling trajectory is obtained.

The direction of the drilling trajectory may be controlled from the earth surface. In that case, the drilling system may comprise downhole control means with at least one sensor, such as a weight-on-bit sensor, a fluid pressure sensor, a rotational speed sensor and the like, as well as a control unit for controlling the steering means on the basis of data detected by said sensor. Preferably, the downhole control means may comprise a memory containing stored trajectory data as well as control means containing preprogrammed control data for controlling the stabilizer radial dimension dependant on the trajectory.

In one aspect the invention relates to a method for operating an abrasive jet drilling system as described before, comprising the steps of:

-   -   making the drill string and abrasive jet drill head rotate,     -   applying a specific data sequence, such as a weight-on-bit         sequence, as a code,     -   detecting said code by the sensor,     -   controlling the radial dimension of the stabilizer on the basis         of said detected code.

Alternatively, the method may comprise the steps of:

-   -   making the drill string and abrasive jet drill head rotate,     -   applying a specific data sequence, such as a weight-on-bit         sequence, as a code,     -   detecting said code by the sensor,     -   controlling the directional action of the stabilizer on the         basis of said detected code.

According to yet another possibility, the method may comprise the steps of:

-   -   making the drill string and abrasive jet drill head rotate,     -   applying a specific data sequence, such as a weight-on-bit         sequence, as a code,     -   detecting said code by the sensor,     -   controlling the eccentricity of the stabilizer on the basis of         said detected code.

The radial dimension of the stabilizer may be adapted while carrying out a drilling operation. According to another possibility, the stabilizer settings may be controlled as a function of a pre-programmed borehole trajectory in combination with downhole measurements while drilling. Such method may thus comprise the steps of:

-   -   making the drill string and abrasive jet drill head rotate,     -   detecting a downhole condition by the sensor,     -   adjusting the stabilizer size on the basis of the stored         trajectory data and the detected downhole condition.

The invention will now be described further by way of example with reference to the embodiments shown in the drawings.

As shown in FIG. 1, an abrasive jet drilling system including an abrasive jet drilling assembly according to the invention comprises a drill string 1 in a borehole 2 in an object. This object is here a subterranean earth formation 5, in particular to provide a borehole for the manufacture of a well for production of mineral hydrocarbons. The drill string 1 is which at its upper end at surface 8 connected to a rotational drive device (not shown, but indicated by arrow 10) and at the other, lower, end to an abrasive jet drill head 16 with jet nozzle 18. The drill string 1 has a passageway 20 for fluid, which is in fluid communication with the jet nozzle, via a passage through the stabilizer means and passage 24 of abrasive jet drill head 16. Furthermore, pump means (not shown) are provided at surface for circulating the drilling fluid from the surface through the drill string 2 to the drill head 16.

The nozzle 18 is obliquely oriented in a recess 17 in an axial area so that the impingement area is located eccentric with respect to the drill axis or rotary axis 21, and in this case rotating the abrasive jet in the hole results in the jet 19 and the impingement area moving along an essentially circular trajectory in the hole. Preferably, the eccentric impingement area overlaps with the centre of rotation, so that also the middle of the bore hole is subject to the erosive power of the abrasive jet.

The jet nozzle 18 is arranged above an optional foot part 29, and the nozzle is inclined relative to the longitudinal direction of the system (drill axis 20) at a nozzle angle of 15-30° relative to the drill axis, but other angles can be used. Preferably the nozzle angle is about 21° which is optimal for abrasively eroding the bottom of the bore hole by axially rotating the complete tool inside the bore hole.

The abrasive jetting drill head in this embodiment moreover comprises a recirculation system for abrasive particles, which is generally indicated as 30, with an inlet 32 in fluid communication with the annulus 33 between abrasive jet drill head 16 and the borehole 2, and an outlet 34 to a mixing chamber 36 arranged at a mixing location 37 of the passageway 24.

The optional foot part 29 forms a front end of the drill head and engages the borehole bottom providing for a distance from the borehole bottom and suitably contains slots for drilling fluid and cuttings to flow via the annulus 33 upwardly. The abrasive jet drill head 18 can for example be a head as described in e.g. WO2008/113843, WO 2008/113844.

In operation, the system works as follows. A stream of drilling fluid including abrasive particles such as steel shot, is pumped from the object's surface (e.g. earth's surface) by a suitable pump (not shown) through the longitudinal passage 20 of the drill string 2. Part or all of the drilling fluid is led to the jet nozzle 18 where an abrasive jet 19 is generated. The abrasive jet is blasted into impingement with the formation. The formation is eroded in the impingement area as a result of the abrasive jet 19 impinging the formation 5, thereby deepening the borehole 2. The positioning and oblique orientation of the nozzle in the axial area provides that a substantially cone-shaped central area of the borehole bottom is created, which centralizes the jet drill head 16, i.e. the head is self-centralizing.

Simultaneously, the abrasive jet is rotated about the rotary axis 20. Thus, the impingement area is moved along a circular trajectory in the hole so that the formation can be eroded at all azimuths at the borehole bottom. By keeping the erosive power of the abrasive jet constant, the formation is eroded evenly on all sides of the hole and consequently the hole is excavated straight. Nevertheless, distortions in the rotating of the excavation tool, or variations in rock formation properties in the hole region, or other causes may result in uneven erosion in the hole. A directional correction may be required by modulating the erosive power to compensating for the unintentional uneven erosion.

As shown in FIG. 1, the trajectory of the borehole 2 is curved. This curvature is obtained by the action of the stabilizer means 40 which is connected to or provided around the drill string 1 immediately above or at a small distance above the drill head 2. Said stabilizer 40 is in contact with the wall of the borehole at 42 under the influence of gravity. The distance between the contact point 42 and the point where the drill head is centralized (in this example at the front end of the foot part 29) can be between 0.1 m and 50 m, such as between 0.5 m and 10 m.

The stabilizer 40 of this embodiment is coaxial with the drill string 1, defining a circular cross-section at the widest radial dimension. It is fixedly arranged around the drill string so that it rotates together with the drill string 1, and it has a diameter smaller than the borehole diameter. The diameter together with the distance from the centralized drill head determines the inclination angle 45 between the drill axis 20 and the borehole axis 48. The better the jetting assembly is centralized at the first point of contact with the borehole above the drill head, the smaller the directional tendency is. Thus, if the first point of contact is at a stabilizer, the outer diameter of this stabilizer determines the building tendency. If the outer diameter of the near-bit stabilizer can be changed while drilling, the building tendency can be controlled while drilling.

For directional drilling operation the inclination angle is normally non-zero, and can for example be between 0.01 and 20 degrees, such as between 0.1 and 5 degrees. The inclination angle in the Figure is positive, i.e. the borehole is deviated upwardly. The invention can however also provide negative inclination angles (deviation towards the vertical), and lateral inclinations, so that directional drilling into any desired spatial direction can be provided.

It shall be clear that the schematic drawing illustrates the various components but does not necessarily represent their relative size correctly. For relatively large inclination angles the diameter of the drill pipe or collar can be limiting, so that the drill pipe or collar 3 between drill head 16 and stabilizer 40 and/or the drill string or tubular above stabilizer 40 has to be sufficiently thin for a desired inclination angle, to ensure that the stabiliser above the bit is touching the borehole wall.

The expressions upper, above, upstream, uphole, lower, below, downstream, downhole, and the like, are used herein with reference to a drill string with jet drill head in a borehole, wherein upper or above is closer to surface than lower or below; and upstream and downstream are with respect to drilling fluid flowing generally downwards through the drill string, and upwards to surface though the annulus with the borehole wall.

The mechanical forces on the drilling system that is based on abrasive jetting are much smaller than is the case for systems based on mechanical rock removal. This has the advantage that the sensors can be located very close to the excavating tool, making early and accurate signal communication possible to the modulation control means. The sensors can for instance be provided in the same chamber as the modulation control means. The control means can comprise a memory for storing trajectory data.

Alternatively, the position and and/or the direction of progress through the formation of the abrasive jet can be determined on the basis of parameters available on the surface 8, including torque on the drill string 2 and azimuthal position of the drill string 2, and axial position and velocity of the drill string 2.

A decision to change or correct drilling direction may also be taken via the operator of the directional system at surface. In case of the signal originating from a down-hole measurement while drilling sensor, a mud-pulse telemetry system or any other suitable data transfer system can be employed to transfer the data to the surface. Via similar means of data transfer a control signal can be sent to the down hole control means triggering a series of control actions required for the desired direction drilling correction.

A thruster (not shown) is advantageously provided for pressing the abrasive jetting system upon the bottom 39 of the hole 2. Best results are obtained when the pressing force is not much higher than what is required to keep the abrasive jet drill head 16 at the bottom, in order to avoid unnecessary wear on the abrasive jet drill head 6, bending of the system, and loss of directional control. Thus, the pressing force is preferably just sufficient to counteract the axial recoil force of the abrasive jet and the friction forces in the thruster and between the abrasive jet system and the hole wall. Typically, the pressing force is well below 10 kN.

A suitable abrasive jet comprises a mixture containing a fluid, such as the drilling fluid, and a certain controlled concentration of abrasive particles. The erosive power of the jet correlates with the total power vested in the abrasive particles entrained in the mixture. This depends on the mass flow rate of abrasive particles and on the square of the velocity of the abrasive particles.

Still referring to FIG. 1, the abrasive particles will be entrained in a return stream of drilling fluid through the excavated hole, running for instance through an annular space 33 between the hole 1 and the drilling system (2,16, 40).

In order to reduce the concentration of abrasive particles to be transported all the way back to the surface, the drilling system, in particular the abrasive jet drill head 16, can be provided with recirculation means 30 arranged to recirculate at least a part of the abrasive particles from the return stream downstream from impingement with the formation, back into the abrasive jet 10 again. The abrasive particles to be recirculated can be mixed with the fresh stream of drilling fluid containing a supply concentration of abrasive particles, for instance in a mixing chamber to which both the fresh stream of drilling fluid and the recirculated abrasive particles are admitted, to obtain a jetting fluid mixture comprising a jetting concentration of abrasive particles. The abrasive particles referred to herein preferably comprise or consist of magnetisable material, i.e. paramagnetic or ferromagnetic material, such as for instance steel shot or steel grit. This will herein also be referred to as “magnetic material” although it does not need to have a permanent magnetization. The recirculation system can comprise a magnet attracting magnetic particles from the drilling fluid flowing upwardly in annulus 33, and conveying the particles via outlet 34 to the mixing chamber 36. Generally suitable recirculation systems are for example described in WO 2002/034653, WO 2005/005766, WO2008/119821, WO 2008/113844. A recirculation system is however optional and not required for the present invention to function.

In an illustrative example, the rotation of the string can typically take 1 sec. In the case a downhole recirculation device is used, the supply concentration of particles pumped through the drill string is typically in the range of 0.1 to 4% by volume, such as 0.4 to 2 vol %, considering steel shot in an aqueous fluid, e.g. water. When a recirculation system is used, the drilling fluid in the abrasive jet may contain a jetting concentration of up to 10% by volume, typically up to 5 vol % of magnetic abrasive particles, and is on average higher than the supply concentration. When there is no recirculation system, the supply concentration via the drill string is typically the same as the jetting concentration, apart from a possible time lag of changes, and can e.g. be in the range of 0.5 to 10 vol %, such as 2-5 vol %, e.g. 3 vol%. Recycle frequency preferably exceeds the rotational frequency of the drill string. The recycle frequency can for example be between between 10 and 40 Hz. The rotation of the drill string, or at least the abrasive jet drill head excavating tool, is typically between 0.3 and 3 Hz.

Referring to FIG. 2, there is shown a drill string 1,3 with jet drill bit at its lower end, and a stabilizer 40 that is concentric and fixed with the drill string, and circumferentially engaging the borehole wall. The stabilizer acts as a centralizer, the drill axis 20 is collinear with the borehole axis 48, i.e. there is no inclination angle. If the stabilizer is of a type that can change its diameter, by lowering the diameter a configuration similar to FIG. 1 in accordance with the invention would be obtained.

FIG. 3 shows an embodiment of the invention similar to FIG. 1, but in which the drill string above the stabilizer is of a narrower diameter than the drill string part 3, so that the stabilizer 40 provides the first point of contact 42. The upper drill string part can also be of a flexible tube. The drill string part 3 can be thicker or also thin drill pipe, or could be a collar containing e.g. a control means and/or a sensor means and/or a communication means.

FIG. 4 shows an embodiment with an eccentric stabilizer 50. It has a non-uniform radial extension around the drill string. The extension in the radial dimension 52 is smaller than in radial dimension 54. The stabilizer conforms to the borehole wall, so that the contact 42 is around the circumference. Here it rotates with the drill string, with the long and short radial dimensions also turning, and therefore the drill axis 48 turns around the borehole axis at the inclination angle. When a nozzle is arranged like in FIG. 1, obliquely in the axial area of the recess 17, and with the eccentricity as drawn, the angle between nozzle and borehole axis is increased by the inclination angle, and a straight, but wider borehole is created. If the nozzle would be turned by 180 degrees to jet against the opposite side of the borehole bottom, a narrower straight borehole would be created. If the eccentric stabilizer would not conform to the borehole, a wider or narrower curved borehole would be drilled.

For larger overgauge hole sizes the bottom profile and the internal conical profile of the distance holder (foot part 29) should be modified to allow for the angle between the drilling assembly and the borehole axis 48.

FIG. 5 is similar to FIG. 4, but the eccentric stabilizer 60 has a cross-section that is adjusted while rotating the drill string to maintain the drill axis 20 geostationary during at rotation. The drill axis 20 has a negative inclination with regard to the borehole axis 48. The stabilizer means can actively controlled. An adjustable eccentric stabilizer can be made as follows: If the stabilizer has four pads that can be moved radially independent of each other, of which one pad is moved outward and one pad moved inward, the axis of the connected drilling assembly is not parallel to the bore hole axis anymore. The pads of an adjustable eccentric stabilizer are adjustable down hole with, for instance, controlled stepper motors or hydraulically operated pistons that are controlled by a down hole control unit, e.g. in collar 3, so that the radial dimensions 62 and 64 remain constant as shown during rotation. Varying the pad positions synchronized with the rotation of the drilling assembly provides directional control in all drilling directions, i.e. both azimuth and inclination control.

In order to establish the current drilling direction through the formation, the system can be provided with a navigational sensor, for instance a measurement while drilling sensor, for providing a signal indicative of the direction under which the making of the hole in the earth formation progresses.

Such a navigational sensor can be provided in the form of one of or a combination of a directional sensor providing a signal indicative of the direction of the device relative to a reference vector; a positional sensor providing a signal indicative of one or more positional coordinates relative to a reference point; a formation density sensor providing information on a distance to a change of formation type or formation content nearby; or any other suitable sensor.

Over longer time scales than a rotation period, a downhole control unit can moreover adjust the adjustable stabiliser settings as a function of a preprogrammed trajectory and down hole measurements while drilling. Also, the instructions can be sent from surface to the control unit, e.g. by data sequences such as a weight on bit sequence, a rotational speed (RPM) sequence, or a hydraulic pressure sequence, e.g. to adjust the magnitude of the inclination or the direction. Based on such transmission of data, one or more of an outer diameter of the stabilizer, and eccentricity, or a directional drilling mode can be set.

It is attractive to combine such options with jet drilling because of the minimal mechanical loading of the drilling assembly compared to mechanical drilling. The mechanical forces on the drilling system that is based on fluid jetting are much smaller than is the case for systems based on mechanical rock removal. This has the advantage that the sensors can be located very close to the jet drill head, making early and accurate signal communication possible to the modulation control means. The sensors can for instance be provided in the same chamber as the modulation control means. 

1. A method of drilling into an object, the method comprising providing a drill string in a borehole in the object, the borehole having at its bottom a borehole axis, the drill string comprising a borehole-centralized jet drill head at its lower end, the jet drill head having a drill axis, and comprising a jet nozzle; providing a stabilizer means for the drill string, the stabilizer means determining an inclination angle between the borehole axis and the drill axis; and generating a fluid jet at the jet nozzle, so as to blast with an erosive power on an impingement area of the borehole, thereby deepening the borehole, and wherein the drill string is rotated while deepening the borehole.
 2. A method according to claim 1, wherein the fluid jet is an abrasive fluid jet and the jet drill head is an abrasive jet drill head.
 3. The method according to claim 1 wherein the jet drill head is a self-centralizing jet drill head.
 4. The method according to claim 1 wherein the stabilizer means is fixed with respect to the drill string while deepening the borehole.
 5. The method according to claim 1 wherein the stabilizer means is an eccentric stabilizer means.
 6. The method according to claim 1 wherein the stabilizer means has a largest diameter that is smaller than the borehole diameter.
 7. The method according to claim 1 wherein at least one radial dimension of the stabilizer means is adjusted in the course of a drilling operation, so as to deepen the borehole along a predetermined trajectory.
 8. The method according to claim 7, wherein the stabilizer means has a cross-section that is adjusted while rotating the drill string.
 9. A jet drilling system for drilling a borehole into an object, comprising a drill string with an jet drill head at its lower end, the drill head being provided with a jet nozzle, a stabilizer means for the drill string at a position above the jet drill head, the stabilizer means protruding radially with respect to adjoining drill string parts so as to provide a point of contact with the borehole wall, and a rotation means for rotating the drill string.
 10. The jet drilling system according to claim 9, further comprising centralizing means for the jet drill head downhole of the stabilizer means.
 11. The jet drilling system according to claim 9 wherein the jet drill head has a front end for engaging the borehole bottom and an axis, and wherein the front end has a recess at least in an axial region.
 12. The jet drilling system according to claim 9 wherein the stabilizer means is adjustable in at least one radial dimension.
 13. The jet drilling system according to claim 12, further comprising a control means for controlling the at least one radial stabilizer dimension.
 14. The jet drilling system according to claim 9 wherein the jet drill head is an abrasive jet drill head. 